The present invention relates to a frequency shift key (FSK) demodulator and particularly, although not exclusively, to an FSK demodulator for a direct conversion paging receiver.
Paging receivers have used a double conversion superheterodyne architecture, but such receivers usually require first and second stage image filters, as well as ceramic filters for use in their channel filter and frequency discriminator circuits. These requirements can make such receivers both expensive and bulky.
Limited output direct conversion receivers, on the other hand, do not require these filters, nor do they require large dynamic range automatic gain control circuits. However, to faithfully demodulate four-level frequency shift keyed signals, it has been necessary to provide these receivers with multibit analogue to digital converters and complex and expensive demodulation circuitry.
In accordance with one aspect of the present invention, an FSK demodulator, for demodulating I and Q output signals provided by a direct conversion receiver arrangement comprises means responsive to said I and Q signals to provide a first signal dependent on the frequency of said I and Q signals and a second signal dependent on the relative phase of said I and Q signals, and a finite impulse response filter means arranged to receive said first and second signals and to provide an output signal representative of data carried by said I and Q signals.
In accordance with another aspect of the present invention there is provided a demodulator arrangement for a direct conversion receiver responsive to four-level frequency-shift keyed received signals to provide phase-quadrature I and Q signals, the demodulator arrangement comprising means to provide a pulse output signal in response to each transition between levels in said I and Q signals, means to provide a phase output signal indicative of the relative phase of said I and Q signals, a finite impulse response filter responsive to said pulse output signals and said phase output signals to provide a pulse-rate signal indicative of the frequency of said I and Q signals, and decoder means responsive to said pulse-rate signal and to said phase output signal to provide demodulated output signals.